1. Field of the Invention
The present invention relates to a current resonant power source apparatus, and particularly, to controlling an output voltage of the current resonant power source apparatus under light load.
2. Description of Related Art
FIG. 2 is a circuit diagram illustrating a current resonant power source apparatus according to a related art. In FIG. 2, a full-wave rectifying circuit RC1 rectifies AC voltage. Both ends of the full-wave rectifying circuit RC1 are connected to a smoothing capacitor C1 and a series circuit of MOSFET switch elements Q1 and Q2. The switch elements Q1 and Q2 are alternately turned on and off. Both ends of the switch element Q2 are connected to a series circuit including a resonant reactor Lr, a primary winding P of a transformer T, and a current resonant capacitor C2.
The transformer T has secondary windings S1 and S2 that are connected in series. A second end of the secondary winding S1 is connected to an anode of a diode D1. A first end of the secondary winding S2 is connected to an anode of a diode D2. Cathodes of the diodes D1 and D2 are connected to a first end of a smoothing capacitor C3. A second end of the smoothing capacitor C3 is connected to a node between a first end of the secondary winding S1 and a second end of the secondary winding S2. The first and second ends of the smoothing capacitor C3 are connected to a detector 11. The resonant reactor Lr may be a leakage inductance of the transformer T.
The detector 11 detects an output voltage of the smoothing capacitor C3 and outputs the detected voltage to an oscillator 13. According to the output voltage of the smoothing capacitor C3, the oscillator 13 changes the oscillation frequency of a frequency signal to output. A comparator CM1 compares the frequency signal from the oscillator 13 with a divided voltage obtained by dividing a voltage of a power source Vcc by resistors R1 and R2, and if the frequency signal is equal to or greater than the divided voltage, outputs a high-level signal. If the frequency signal is smaller than the divided voltage, the comparator CM1 outputs a low-level signal.
An inverter IN1 inverts the output of the comparator CM1, to turn on/off the switch element Q2. A high-side driver 12 turns on/off the switch element Q1 according to the output of the comparator CM1.
Operation of the current resonant power source apparatus according to the related art having the above-mentioned configuration will be explained. When the switch element Q1 is turned on, a current passes clockwise through a path extending along RC1, Q1, Lr, P, C2, and RC1. This current is a resultant current of an exciting current passing through an exciting inductance Lp on the primary side of the transformer T and a load current supplied through the primary winding P, secondary winding S2, diode D2, and capacitor C3 to an output terminal OUT and a load. The exciting current mentioned above is a sinusoidal resonance current of the “reactor Lr+exciting inductance Lp” and current resonant capacitor C2. Part of the sinusoidal resonance current is observed as a triangular current because a resonance frequency is lower than an ON period of the switch element Q1. The load current mentioned above is a sinusoidal resonance current involving a resonance element of the reactor Lr and current resonant capacitor C2.
When the switch element Q1 is turned off, energy accumulated in the transformer T by the exciting current causes the “reactor Lr+exciting inductance Lp”, the current resonant capacitor C2, and a voltage resonant capacitor Crv (not illustrated) appearing between each end of the switch element Q2 to demonstrate a quasi-voltage-resonance. At this time, a resonance frequency of the voltage resonant capacitor Crv whose capacitance is small is observed as a voltage across the switch elements Q1 and Q2. Namely, when the switch element Q1 is turned off, the current of the switch element Q1 shifts to the voltage resonant capacitor Crv. When the voltage resonant capacitor Crv is discharged to zero volts, the current shifts to an internal diode of the switch element Q2. This causes the energy accumulated in the transformer T by the exciting current to charge the current resonant capacitor C2 through the internal diode of the switch element Q2. During this period, the switch element Q2 is turned on to realize the zero-volt switching of the switch element Q2.
When the switch element Q2 is turned on, the current resonant capacitor C2 serves as a power source to pass a current counterclockwise through a route extending along C2, P, Lr, Q2, and C2. This current is a resultant current of an exciting current passing through the exciting inductance Lp of the transformer T and a load current supplied through the primary winding P, secondary winding S1, diode D1, and smoothing capacitor C3 to the output terminal OUT and load. The exciting current mentioned above is a sinusoidal resonance current of the “reactor Lr+exciting inductance Lp” and current resonant capacitor C2. Part of the sinusoidal resonance current is observed as a triangular current because a resonance frequency is lower than an ON period of the switch element Q2. The load current mentioned above is a sinusoidal resonance current involving a resonance element of the reactor Lr and current resonant capacitor C2.
When the switch element Q2 is turned off, energy accumulated in the transformer T by the exciting current causes the “reactor Lr+exciting inductance Lp”, the current resonant capacitor C2, and the voltage resonant capacitor Crv to demonstrate a quasi-voltage-resonance. At this time, a resonance frequency of the voltage resonant capacitor Crv whose capacitance is small is observed as a voltage across the switch elements Q1 and Q2. Namely, when the switch element Q2 is turned off, the current of the switch element Q2 shifts to the voltage resonant capacitor Crv. When the voltage resonant capacitor Crv is charged to the output voltage of the smoothing capacitor C1, the current shifts to an internal diode of the switch element Q1. This causes the energy accumulated in the transformer T by the exciting current to be regenerated to the smoothing capacitor C1 through the internal diode of the switch element Q1. During this period, the switch element Q1 is turned on to realize the zero-volt switching of the switch element Q1.
FIG. 3 illustrates waveforms at different parts of the current resonant power source apparatus of the related art under light load. In FIG. 3, Id (Q1) is a drain current of the switch element Q1, I(P) is a current passing through the primary winding P, V(C2) is a voltage across the current resonant capacitor C2, Vds(Q2) is a drain-source voltage of the switch element Q2, V(P) is a voltage across the primary winding P, V(D1) is a voltage across the diode D1, and V(D2) is a voltage across the diode D2.
The current resonant power source apparatus of the related art alternately turns on/off the switch elements Q1 and Q2 at a duty of 50% and controls a switching frequency, thereby controlling an output voltage. As illustrated in FIG. 3, the voltage V(C2) of the current resonant capacitor C2 repeats an up-down symmetrical charging and discharging actions around a half of a voltage V (C1) of the smoothing capacitor C1. As a result, the primary winding P of the transformer T generates the voltage V(P) to generate voltages on the secondary windings S1 and S2. These voltages are rectified through the diodes D1 and D2, to provide an output voltage.
Related arts concerning the current resonant power source apparatus include Japanese Unexamined Patent Application Publications No. 2013-78228 (Patent Literature 1) and No. H07-135769 (Patent Literature 2).